The present invention relates to a method for fabricating metal wirings used for flat panel displays such as liquid crystal displays (LCDs), plasma display panels (PDPs), electrochromic displays (ECDs) and electroluminescent displays (ELDs), printed wiring boards using ceramic boards, and other various fields.
Conventionally, in a flat panel display typified by LCDs, normally, display material such as liquid crystals is held between a pair of substrates and a voltage is applied to this display material. In this case, electrical wiring lines are arrayed on at least one of the substrates.
For example, in the case of an active matrix drive type LCD, on one of a pair of substrates constituting part of a display unit, gate electrodes and data electrodes are disposed in a matrix shape, and thin film transistors (TFTs) and pixel electrodes are disposed at individual intersections of these electrodes. Normally, these gate electrodes and data electrodes are made of a metal material such as Ta, Al or Mo, and deposited by a dry film formation process such as sputtering process.
In such flat panel displays, in an attempt to implement larger areas and higher definitions, the drive frequency would increase while the electric wiring resistance as well as the parasitic capacitance would increase. As a result of this, delay of driving signals would come up as a large problem.
Thus, in order to solve the problem of the delay of driving signals, there have been made attempts to use Cu (bulk resistivity: 1.7 xcexcxcexa9xc2x7cm), which is lower in electrical resistance, instead of Al (bulk resistivity: 2.7 xcexcxcexa9xc2x7cm), xcex1-Ta (bulk resistivity: 13.1 xcexcxcexa9xc2x7cm) or Mo (bulk resistivity: 5.8 xcexcxcexa9xc2x7cm), which are conventional wiring materials. For example, xe2x80x9cLow Resistance Copper Address Line for TFT-LCDxe2x80x9d (Japan Display ""89, pp. 498-501) discloses discussion results on a case of using Cu as the gate electrode material of TFT-LCDs. According to this literature, it is expressly described out that because a Cu film deposited by sputtering process is poor in adhesion with the ground glass, a metal film of Ta or the like needs to be interveniently provided as a ground film in order to enhance the adhesion.
However, in the case of the wiring structure in which a metal film of Ta or the like is provided as the ground, dry formation processes and etching processes would be involved individually for the Cu film and the ground metal film of Ta or the like, causing a process increase and leading to a cost increase, as a disadvantage.
Thus, in Japanese Patent Laid-Open Publication HEI 4-232922, there has been proposed a method in which while a transparent electrode made of ITO (Indium-Tin-oxide) or the like is used as a ground film, a metal film of Cu or the like is formed by plating technique on the ground film. In this technique, it is expressly described that since the plated metal can be formed selectively only on the ITO film, the patterning process is required only for the ITO film of the transparent electrode so that Cu wiring can effectively be formed even for large areas. The publication also describes that a metal film of Ni or the like having good adhesion with the ITO film is interveniently provided between the ITO film and Cu wiring.
On the other hand, in addition to the electrical wiring fabricating method described in Japanese Patent Laid-Open Publication HEI 4-232922, there have been proposed electrical wiring fabricating methods in which a film of Ni, Au, Cu or other metal is formed on a patterned ITO film by plating technique for various purposes such as the process reduction for the active matrix substrate, lower resistance of the transparent conductive film in simple matrix type LCDs or the like, and improvement solder wettability on the ITO film (see, e.g., Japanese Patent Laid-Open Publications HEI 2-83533, HEI 2-223924, HEI 1-96383, SHO 62-288883).
However, in the case where the Cu/Ta lamination film is formed by sputtering process, i.e., where both the Cu film for lower resistance and the ground metal film intended to improve the adhesion with the Cu film are formed by vacuum deposition equipment, individual film deposition processes are involved for the Cu film and the ground metal film, respectively, causing a process increase and leading to a cost increase, as a disadvantage. Also, individual etching processes are involved for the Cu film and the ground metal film, respectively, causing a process increase and leading to a cost increase, as a disadvantage.
Also, in the electrical wiring fabricating method in which ITO is used for the ground metal film, because the metal film is formed by wet formation technique while the ITO film is formed by vacuum deposition equipment for sputtering process, vapor deposition process or the like, enough cost reduction effect cannot be obtained, resulting in a problem that large-scale substrates cannot be easily managed.
Therefore, an object of the present invention is to provide an electrical wiring fabricating method capable of fabricating the electrical wiring with low cost without using any vacuum deposition and managing large-scale substrates.
In order to achieve the above object, the present invention provides a method for fabricating metal wirings, comprising the steps of:
forming a ground resin film by applying a resin onto an insulating substrate;
patterning the ground resin film; and
forming a low-resistance metal film selectively on the patterned ground resin film by a wet film formation technique.
According to this invention, the ground resin film can be formed by spin coating, like resist or the like. The low-resistance metal film provided thereon can be formed selectively on the ground resin film by a wet film formation technique. Therefore, the need for vacuum deposition equipment, etching equipment or the like is eliminated.
As a result, it becomes possible to form metal wirings without using any vacuum deposition equipment, thus allowing a considerable cost reduction to be achieved as compared with the case where electrical wirings are formed by the method shown in the prior art example.
Also, since the ground film is made of resin, the film having good adhesion can be easily formed on the insulating substrate.
Further, since a wet film formation technique is used for film formation, the film formation can be achieved only by immersing the substrate into a solution, thus easily coping with large-scale substrates.
The wet film formation technique herein referred to is a technique that film formation is done by immersing the substrate into a solution without using any vacuum equipment, the technique being exemplified by plating process, electrolytic process, dip coating process, coating process or the like. In addition, such a film formation technique as shown in later-described Japanese Patent Laid-Open Publication HEI 10-245444 is also included in the scope of the wet film formation technique.
In one embodiment, the ground resin film is made of a photosensitive resin that can be patterned by exposure and development.
According to this embodiment, in addition to the foregoing effects, it becomes possible to easily form a high-definition film, as in the case of photoresist that is currently used. By using such a resin as those used for printed wiring boards, the ground resin film can be provided as a ground film that allows a single low-resistance metal film of, for example, Cu to be formed with good adhesion.
In one embodiment, the low-resistance metal film is a single layer film containing any one of Cu, Ni and Au or a multilayer film containing at least one of these single layers.
According to this embodiment, the low-resistance metal film is made of Cu, which has characteristics of low resistivity (bulk resistivity: 1.7 xcexcxcexa9xc2x7cm) and long life against electromigration, thus optimum as a wiring material.
Also, even with low adhesion between Cu and ground resin, low-resistance wirings of good adhesion with the ground resin can be achieved by using Ni of good adhesion as the ground and forming Cu/Au or the like thereon.
In one embodiment, the ground resin film is made of polyimide.
Since polyimide is superior in heat resistance and chemical resistance among resins, using polyimide as the ground resin as in the metal wiring fabricating method of this embodiment allows a manufacturing method to be chosen from a wide variety of methods for the processes subsequent to the formation of the ground resin film.
For example, when plating process is used as the wet film formation process for forming the low-resistance metal film, the plating solution is in many cases strong alkali or strong acid. Therefore, high chemical resistance is effective for that process.
Also, polyimide, because of its high heat resistance, allows the margin for other film formation processes to be widely taken. For example, while the process maximum temperature for normal amorphous liquid crystals is about 350xc2x0 C., polyimide has a heat resistance of about 400xc2x0 C. (polyimide is normally thermally cured at about 350xc2x0 C.; thermal decomposition temperature of polyimide is, in many cases, not less than 450xc2x0 C.). Therefore, unlike the cases where other resins are used, there is no need of lowering the process temperature. The unnecessity of process change prevents occurrence of failures that would be involved in necessity of process changes, thus giving a large advantage for manufacture of products.
In addition, thermal resistance temperature of other resins is about 200xc2x0 C. for normal resist that is used for liquid crystals, and not more than 250xc2x0 C. for acrylic resin.
Also, some photosensitive polyimides have a resolution of L/S=5 xcexcm or more, which is a satisfactory resolution as the ground material of the low-resistance metal for forming metal wirings.
Also, plated copper on polyimide has been already put into practical use in the field of printed boards or the like, thus satisfactory as the ground film in view of using plating process as the wet film formation technique.
In one embodiment, plating is used as the wet film formation technique, and the ground resin contains a plating catalyst.
According to this embodiment, since the plating catalyst is contained, it becomes possible to use a resin to which it is hard to selectively impart the plating catalyst.
Also, in the case of a resin that allows the plating catalyst to be easily imparted thereto, the step of imparting the catalyst during the plating process can be omitted, thus allowing process reduction to be easily achieved, as an advantage.
One embodiment further comprises a step for, before the step of forming the low-resistance metal film, modifying a surface of the ground resin film.
According to this embodiment, the surface of the ground resin film is modified, thereby forming asperity on the surface of the ground resin film. By the asperity formed on the surface of the ground resin film, the adhesion between the ground resin film and the low-resistance metal film can be improved to such an extent as could not be obtained by the catalyst imparting method used as a pre-processing for conventional plating process.
Also, as the surface of the modified ground resin film is highly capable of adsorbing metal ions, reducing these metal ions allows a metal layer to be obtained on the surface of the ground resin film. That is, a metal film that acts as a catalyst in the process of forming the low-resistance metal film can be formed selectively on the ground resin film without adding any patterning process such as photolithography process.
One embodiment further comprises a step for, after the step of modifying the surface of the patterned ground resin film, forming on the surface-modified ground resin film a metal layer serving as a catalyst in the process of forming the low-resistance metal film by the wet film formation technique.
According to this embodiment, a metal layer is formed on the ground resin film. This metal layer acts as a catalyst in the process of forming the low-resistance metal film on the ground resin film by, for example, electroless plating process as a wet film formation process. Since the metal layer is easily formed on the modified ground resin film as described above, resin materials to which the plating catalyst is hard to selectively impart become usable as the ground resin, thus allowing the material of the ground resin film to be chosen from a wider variety of materials, so that the material cost of the ground resin becomes lower. Also, since the step of imparting the catalyst in the plating process can be omitted even with the use of a resin to which a plating catalyst is easily imparted, the metal wiring manufacturing processes can be reduced.
In one embodiment, the step of forming the metal layer acting as a catalyst in the process of forming the low-resistance metal film by the wet film formation technique comprises the steps of:
making metal ions adsorbed onto the surface-modified ground resin film; and
reducing the metal ions.
According to this embodiment, after metal ions are adsorbed to the ground resin film, the metal layer is formed by reducing the metal ions. That is, without using any dry film deposition technique or etching technique, the metal layer is formed selectively on the ground resin film with ease and lower cost. As a result, the fabrication of the metal wirings is facilitated while the fabrication cost for the metal wirings is reduced.
In one embodiment, the step of modifying the surface of the patterned ground resin film is a process using KOH (Potassium hydroxide).
According to this embodiment, even when the ground resin film is made of a material having strong chemical resistance such as polyimide, an etching process is done with KOH. Therefore, asperity is formed on the surface of the ground resin film, so that a good adhesion of the ground resin film with the low-resistance metal film can be obtained and that the metal ions can be adsorbed enough.
In one embodiment, the metal ions to be adsorbed onto the surface-modified ground resin film are any one of Cu, Ag and Pd ions.
According to this embodiment, these metal ions serve as a catalyst in the process of forming the low-resistance metal film by electroless plating as an example of the wet film formation technique, there is no need of imparting any catalyst for the plating process, so that the process for forming the low-resistance metal film can be reduced. Also, all these metal ions serve as a plating catalyst for the process of forming a low-resistance metal film of, for example, Cu on the ground resin film, the metal ions can be selected depending on the kind of the ground resin film or the conditions for the plating process.
In one embodiment, the step of reducing the metal ions is a process in which ultraviolet rays are irradiated to places where the low-resistance metal film is to be formed, by which the metal ions are selectively reduced.
According to this embodiment, the metal ions are reduced by irradiating ultraviolet rays, there is no need for a reducing agent, and so the liquid waste processing that would be involved in the use of a reducing agent is no longer necessary. Thus, the material cost for the reducing agent or the like and the liquid waste processing cost are reduced, so that the fabrication of metal wirings become easier and lower in price.
Further, the metal layer that serves as a catalyst in the process of forming the low-resistance metal film can be selectively formed by selectively reducing the metal ions of the ground resin film with the use of, for example, a mask. In this case, it becomes possible to form low-resistance metal film of different patterns on the patterned ground resin film.
The ground resin film to be formed in the present invention is preferably formed into a thickness of 0.05-0.5 xcexcm. For example, in the case where metal wirings for an active matrix drive type LCD or the like are formed according to the present invention, if the ground resin film is thicker, there would occur such problems as disconnections at wiring jumping portions or occurrence of cracks at edge portions. With regard to the metal wirings of this active matrix drive type LCD, the total thickness of the metal wirings is desirably 0.5-0.8 xcexcm at most, and therefore the thickness of the ground resin film is desirably not more than 0.5 xcexcm at most. However, if the ground resin film is too thin, there would occur such problems as lowered adhesion between ground resin and insulating substrate, or dissipation of the ground resin film due to uniformities of etching depth in the etching process. Therefore, the thickness of the ground resin film is desirably not less than 0.05 xcexcm.